1. Field of the Invention
This invention relates to electromagnetic detection systems and more particularly it concerns novel arrangements which provide improved performance from such systems.
2. Description of the Prior Art
French Pat. No. 763,681 issued on Feb. 19, 1934 to Pierre Arthur Picard shows one type of electromagnetic detection system upon which the present invention provides improvements. As disclosed in that patent, an article to be detected, e.g., a library book, is provided with a target or marker of a special saturable magnetic material, such as permalloy. A large coil is arranged near a doorway or other egress passageway leading from the area where the article to be detected is kept. This coil is energized with an alternating electrical signal so that it produces a corresponding alternating interrogation magnetic field in an interrogration zone in the region of the doorway. When the book bearing the permalloy marker passes through the alternating interrogation magnetic field, as when it is carried out through the doorway, the permalloy marker converts a portion of the alternating interrogation magnetic field energy to other alternating magnetic fields at various harmonics of the frequency of the fundamental or interrogation magnetic field. Selected ones of these harmonics are detected in the receiver; and when these harmonics are detected an alarm is sounded.
Another prior art electromagnetic detection system is shown and described in U.S. Pat. No. 3,500,373. In that system electromagnetic waves are generated at swept frequencies in an interrogation zone and a target, comprising a resonant circuit tuned to one of the frequencies, causes electromagnetic disturbances which are detected.